1. Field of the Invention
The present invention relates generally to an apparatus and method for forming laminated structures and more particularly to an apparatus and method for fabricating a plurality of multilayer ceramic products simultaneously with uniform uniaxial compression.
2. Problem to be Solved
Multilayer ceramic (MLC) structures are used in the production of electronic substrates and devices. The MLCs can have various layering configurations. For example, a MLC circuit substrate may comprise patterned metal layers which act as electrical conductors sandwiched in between ceramic layers which act as a dielectric medium. The ceramic layers have tiny holes or VIA holes. Prior to lamination, the VIA holes are filled with conductive metal paste and sintered to form VIAS which provide the electrical connection between the layers. In addition, the substrates may have termination pads for attaching semiconductor chips, connector leads, capacitors, resistors, etc.
Generally, conventional ceramic structures are formed from ceramic green sheets which are prepared from a slurry of ceramic particulates, thermoplastic polymer binders, plasticizers, and solvents. This composition is spread or cast into ceramic sheets or slips from which the solvents are subsequently volatilized to provide coherent and self-supporting flexible green sheets. After punching, stacking and laminating, the green sheets are fired at temperatures sufficient to burn-off or remove the unwanted polymeric binder resin and sinter the ceramic particulates together into a densified ceramic substrate. The present invention directed to the lamination steps of this process.
In the MLC packaging industry it is very common to use uniaxial hydraulic lamination presses to provide pressure necessary for laminating the personalized or punched, stacked green sheets in order to obtain layer to layer connectivity. Designing appropriate lamination cycles or algorithms, which include controlling pressure, temperature and lamination time for a given green sheet and metal paste combination, is necessary for obtaining predictable product quality.
Generally, lamination methods include the control of variables such as pressure, temperature and time. These controls ensure layer to layer bonding with minimal pattern distortion, uniformity in product thickness and material compression. Lamination pressure and temperature is used to adjust the product's shrinkage along the x-y axis during sintering, irrespective of the product's x-y-z dimensions. In particular, the lamination pressure plays an important role in shrinkage control. The lamination temperature depends on the glass transition temperature T.sub.g of the binder system which adds strength for pre-fire handling. These temperatures vary and are typically between 1250.degree. C. and 1560.degree. C. depending upon the raw material used to form the green sheets. Some green sheet raw materials include alumina, glass ceramics, aluminum nitride, borosilicates, etc.
Green sheet technology has advanced so that the mechanical properties of the sheets can be reproduced consistently; however, the green sheet thickness and green sheet density can only be controlled within certain accuracy and tolerance. For example, the thickness variation in a 0.02043 cm (0.008 in.) green sheet could be +/-0.00127 cm (0.0005 in.) and the green sheet density can vary from 1.52 g/cm.sup.3 (0.05491 lb/in.sup.3) to 1.65 g/cm.sup.3 (0.05961 lb/in.sup.3) on a nominal green sheet density of 1.55 g/cm.sup.3 (0.05600 lb/in.sup.3).
In an alumina system, it is preferred that approximately 11% nominal compression be created in the Z axis during lamination. The x-y axis shrinkage and camber are kept under acceptable spread in sintering by controlling the laminate compression. This is achieved consistently in uniaxial laminations as long as the green sheet density is within acceptable range as described above. The variation in sheet thickness produces various thicknesses of MCM packages, i.e. thicker or thinner, at a constant lamination pressure. The lamination pressure is chosen depending upon the final x-y substrate size and end laminate density. A typical lamination pressure is 27,580 kPa (4,000 psi). The green laminate, or unfired stack of green sheets, density is assured as long as the lamination pressure is the same. This starting laminate density essentially controls the x-y axis shrinkage in sintering for a given sinter cycle.
The apparatus and process of single frame lamination on a platen is simple and well understood and therefore, the lamination process can be easily controlled. However, single frame lamination technology is time consuming. The cost of lamination can be substantially reduced by improving the throughput of the press or tooling. Throughput can be improved if multiple frames per tool were used in a single lamination cycle.
FIG. 1 illustrates the prior art lamination apparatus which uses a single frame per platen per lamination cycle and a hydraulic lamination press (not shown). The lamination frame 22 and green sheet stack 24 containing at least one green sheet 25 are placed on a first platen 26a of a hydraulic press. The first platen 26a moves upward such that the lamination punch 30 comes in contact with a lamination plate 28b on the stack 24. The lamination punch is secured to a second platen 26b. The hydraulic press applies the required pressure, shown by arrow 34 to the first platen 26a which is transferred to the green sheet stack 24. Typically, the platens 26a,b are heated to a temperature above the glass transition temperature T.sub.g of the polymer in the green sheet formulation; however, a cold lamination process may also be used. The temperature and pressure are controlled for a predetermined time ensuring the bonding of the green sheets and the required laminate compressibility.
While the lamination of multiple products can be achieved in a single frame, physical limitations exist due to thickness of the product, required temperature uniformity, etc.
U.S. Pat. No. 5,163,363 addresses a device to distribute pressure equally to stacks during sintering and to prevent product warpage during densification. A very low force, on the order of 55.16 to 68.95 kPa (8 to 10 psi), is applied during this process. The device is suitable for sintering; however, it is not suitable for lamination because it can not work at the high pressure on the order of 6,895 to 48,265 kPa (1,000 to 7,000 psi), which is necessary for lamination.
The prior art lamination process could take several minutes and handles only single frame lamination on a platen. The procedure is slow and therefore, it is necessary to enhance the productivity of the lamination process to reduce the manufacturing cost by increasing the number of frames per platen during each lamination process and also provide a way to laminate substrates of various thicknesses.
Bearing in mind the problems and deficiencies of the prior art it is therefore an object of the present invention to provide a novel method and apparatus for laminating plurality of products of various thicknesses in a single lamination cycle.
It is another object of the present invention to provide an apparatus and a method that will ensure uniform pressure across plurality of stacked products.
A further object of the present invention is to provide an apparatus and method that will ensure uniform pressure across plurality of stacked green sheets used to produce multilayer ceramic laminates.
It is yet another object of the present invention to have an apparatus and a method for laminating a plurality of stacked green sheets used to produce multilayer ceramic substrates that is very economical.
It is yet another object of the present invention to provide an apparatus and a method for laminating a plurality of stacked green sheets used to produce multilayer ceramic substrates that is predictable and repeatable.
It is yet another object of the present invention to laminate several stacked green sheets products simultaneously with a single lamination cycle.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.